1. Field of the Invention
The present invention relates to a technique for improving the accuracy of color conversion profile used for printing.
2. Description of the Related Art
The image output devices such as displays and printers usually employ the color image data which expresses in tone the color of each pixel in term of specific color components. The image data specify colors by means of any of color spaces, such as RGB color space that uses three colors of R (red), G (green), and B (blue), and CMY color space that uses C (cyan), M (magenta), and Y (yellow), including lc (light cyan), lm (light magenta), DY (dark yellow), and K (black). These colors are usually device-dependent colors inherent in individual image output devices. One way to permit various image output devices to produce identical colors is by using a color-correcting LUT (look-up table) that specifies correspondence between colors for one device and colors for another device.
The color-correcting LUT should specify the correspondence for all the colors that can be produced by any image output device; however, this is impractical from the stand-point of memory capacity and work require to prepare each color-correcting LUT. It is common practice to specify the correspondence for typical colors in a specific number and calculate the correspondence for any other arbitrary colors by interpolation. In other words, common practice is not to perform colorimetry on a very large number of colors but to cause the image output device to produce colors (to an extent for practical colorimetry) and perform colorimetry on these colors, thereby specifying the color-correcting LUT for typical colors in a specific number.
Before preparing the color-correcting LUT, it is necessary to determine the colors in a specific number on which colorimetry is performed or to determine a plurality of lattice points in the color space. One conventional way to determine lattice points is by color separation. The procedure for color separation consists of prescribing cubic lattice points in the CMY space and transforming three colors of CMY at each lattice point into six colors of CMYKlclm according to a special transforming rule, thereby determining lattice points with components of ink colors.
The above-mentioned conventional method for determining lattice points is unable to sufficiently improve the accuracy of color conversion in the color-correcting LUT to be prepared eventually. In other words, the correspondence between CMY and ink color components, which is determined by color separation as mentioned above, conforms to the rule of transformation; however, the arrangement of lattice points may be distorted after color separation because no consideration is given to the smoothness of the arrangement of lattice points in the color space. If color transformation is carried out by referencing the distorted arrangement of lattice points, there occur some portions with locally poor accuracy at the time of interpolation. Particularly, this is an instance where sufficient smoothing is not achieved when R ink or V (violet) ink is used. Consequently, there has been a demand for a technique to select as many lattice points as possible and optimize them to minimize distortion in the arrangement of lattice points while satisfying the rule of color separation. In addition, there is an instance where the density of lattice points in the color space should desirably be higher in one region than another (for high color conversion accuracy) or varies from one region to another according to ink characteristics. In this case, however, it was impossible to consider the local arrangement of lattice points if the arrangement of lattice points is smoothed throughout the entire color space.